Swedish patent specification No. 333,207 discloses a device in burners, in particular rotary burners, in which device the air necessary for the combustion is supplied in the form of approximately concentric streams through a primary air nozzle extending about the nozzle of the burner and movable together therewith, and through a secondary air nozzle extending about the primary air nozzle. The primary air nozzle is connected to a primary air fan by the intermediary of a primary air supply duct, whereas the secondary air nozzle is connected to a secondary air fan by the intermediary of a secondary air supply duct which, in this instance, is in the form of a combustion chamber mounted on the heater. In this prior art device, the burner nozzle and the primary air nozzle are shiftable relative to the secondary air nozzle in that the primary air nozzle connected to the burner nozzle is united, by means of a telescope member or bellows member, with the primary air supply duct fixedly mounted on the combustion chamber. This prior art device makes it possible for the burner nozzle to be given optimum adjustment in relation to the secondary air nozzle for thereby shaping the flame. However, the prior art device is possessed of many disadvantages, since, on dismantling of the burner for service or a similar operation, the entire heater must be turned off, it being necessary, for removal of the burner, to dismantle parts of the primary air supply duct fixedly mounted on the combustion chamber. As a result, free communication will be established between the interior of the furnace and the ambient atmosphere. Thus, in this prior art device, the primary air supply duct must be of the divisible type, with all of the complications which this entails. Moreover, the weight of this prior art device is unnecessarily great as a result of this arrangement, which is a disadvantage in particular when the burners are placed such that their axes of rotation are vertical. Such a vertical assembly is very common in large water heaters.